1. Field of Invention
The present invention relates generally to electrically switchable polymer-dispersed liquid crystal holographic materials, and more specifically to switchable polymer-dispersed liquid crystal materials suitable for switchable optical coupling and reconfigurable optical interconnects.
2. Description of Related Art
Typical state-of-the-art holographic materials do not have an electro-optical nature which can be exploited for real time control of their optical properties. That is, once the hologram is fixed, its optical characteristics cannot be changed. Thus, it is seen that there is a need for materials that can record volume holograms with properties that can be electrically controlled.
Liquid crystals have long been utilized in the prior art for their ability to change their optical orientation in the presence of an electric field. Additionally, liquid crystals can dramatically increase the diffraction efficiency of a volume hologram of which they are a part. Together, these properties offer the very desirable possibility of electrically switching the diffraction efficiency of volume holograms for use in a wide variety of optical information processing and display applications.
The prior art has attempted to combine the properties of liquid crystals with holograms by a variety of methods. Unfortunately, most of these prior art devices are complex to manufacture and are not successful at offering all the advantages of volume holographic gratings.
One approach for combining the advantages of liquid crystals with volume holographic gratings has been to first make a holographic transmission grating by exposing a photopolymerizable material with a conventional two-beam apparatus for forming interference patterns inside the material. After exposure, the material is processed to produce voids where the greatest amount of exposure occurred, that is, along the grating lines, and then, in a further step, the pores are infused with liquid crystals. Unfortunately, these materials are complex to manufacture and do not offer flexibility for in situ control over liquid crystal domain size, shape, density, or ordering.
Polymer-dispersed liquid crystals (PDLCs) are formed from a homogeneous mixture of prepolymer and liquid crystals. As the polymer cures, the liquid crystals separate out as a distinct microdroplet phase. If the polymer is a photopolymer, this phase separation occurs as the prepolymer is irradiated with light. If a photopolymerizable polymer-dispersed liquid crystal material is irradiated with light in a suitable pattern, a holographic transmission grating can be made inside the cured polymer comprising gratings of cured polymer separated by phase-separated liquid crystals. The prior art has attempted to employ polymer-dispersed liquid crystal materials for writing volume gratings, but, despite a variety of approaches, has not been able to achieve high efficiency in the Bragg regime, high density (small grating spacing) capability, or low voltage (<100 Vrms) switching for films in the range of 15 microns thickness. The inability to make an electrically switchable volume hologram that can be switched at voltages less than 100 volts has been a particular deficiency in the prior art in that lower voltages are necessary to be compatible with conventional display and information processing technology.
It is, therefore, a principal object of the present invention to provide an improved polymer-dispersed liquid crystal system suitable for recording volume holograms.
It is a particular object of the present invention to provide a polymer-dispersed liquid crystal system that has a fast curing rate to produce small liquid crystal droplets, particularly in the range of 0.01–0.05 microns, for greater clarity of any resulting film and for writing finer gratings.
It is another object of the present invention to provide a single-step, fast holographic recording material.
It is a further object of the present invention to provide electrically switchable volume holograms that can be switched at voltages less than 100 volts.
It is also an object of the present invention to provide an improved polymer-dispersed liquid crystal system suitable for recording reflection gratings, including, in particular, switchable reflection gratings.
It is also an object of the present invention to provide an improved polymer-dispersed liquid crystal system suitable for recording subwavelength gratings, including, in particular, switchable subwavelength gratings.
These and other objects of the present invention will become apparent as the description of certain representative embodiments proceeds.